The affinity of oxygen for the engineering alloys used in the boiler water industry is the cause of many corrosion phenomena. This is a complex process that not only depends on the amount of oxygen, but also on factors such as the water chemistry and metallurgy. For example, the presence of other species in the water could turn oxygen into an aggressive corrosive force, or could render the metallurgy passivated. Other important factors are temperature, pressure, fluid velocities and operational practices. While oxygen might be the primary or essential component in the corrosion process, it might not be the only one.
The conventional means for reducing oxygen corrosion in water systems is to remove most of the molecular dissolved oxygen by mechanical and chemical means. The vast majority of the dissolved oxygen is reduced into the ppb regime by the use of mechanical deaeration. Here the water is typically heated to above boiling temperature in a vented vessel. The solubility of the dissolved oxygen in this water decreases as the temperature increases. Flow-dynamics and operational issues particular to deaerators leave parts per billion of dissolved oxygen in the water. The chemicals used to reduce the dissolved oxygen values further to reproducibly low and constant values are called oxygen scavengers. Many of these scavengers also function as passivating corrosion inhibitors. Deaerators do not always work perfectly. If they did, a pure scavenger might never be needed, although a chemistry that enhances metal passivation would be a positive addition. So in some cases, the oxygen scavenger is added as an insurance policy against the possibility that the deaerator might malfunction. The scavenger can also be added to combat air in-leakage.
Traditionally, the amount of oxygen scavenger fed to the boiler feedwater has been based on the amount of dissolved oxygen in the feedwater plus some excess amount of scavenger. The amount of excess scavenger fed is based on the desired residual scavenger concentration in the boiler feedwater or boiler water itself, which is a function of the excess concentration of scavenger and boiler cycles. There are several problems with this feed control scheme. The first is that there is no active control of the scavenger feed rate. High oxygen conditions could exist for long periods of time before a decrease in scavenger residual occurs and corrective action is taken. A second issue is that the presence of residual scavenger in the boiler water simply does not mean that the system is being treated satisfactorily. Depending on the conditions (i.e. low temperature or short residence time) it is possible to have both high oxygen concentrations and sufficient scavenger in the feedwater at the same time. When this oxygen rich feedwater reaches the boiler the oxygen is flashed off with the steam leaving the unreacted scavenger in the boiler water. In the extreme case this would result in unacceptably high dissolved oxygen levels in the pre-boiler and condensate systems while having the expected residual concentrations of oxygen scavenger in the boiler itself.
In certain high-pressure boilers (once through) that use ultra-high purity water, a different approach has been taken. No oxygen scavengers are used. In fact small amounts of molecular oxygen are deliberately added to the feedwater. Oxygen, the oxidant, acts as the passivating agent for carbon steel under carefully controlled conditions of boiler water chemistry. Oxygen concentrations used are much less than the air saturated (8 ppm DO) values, thus some deaeration is used. It is often easier to deaerate, to some extent first, prior to adding a controlled amount of oxygen. Accordingly, there is an ongoing need for effective methods for controlling feed of oxygen or oxygen scavengers hot water systems.